The present invention relates to a plastics processing machine for producing fiber-containing thermoplastics, and to a scale assembly for use in a plastics processing machine.
In general, mechanical properties of thermoplastics can be enhanced by adding fiber material, in particular glass fibers. The glass fibers are stored on spools, drums or as so-called rovings, and then introduced via an inlet port into the extruder for plasticizing the thermoplastic pellets. Rotation of the extruder screw draws the glass fibers into the extruder interior, which are then comminuted during following screw rotations and evenly spread in the plasticized material. A uniform quality of the produced plastic products can be assured by maintaining a constant quantitative ratio between plastic pellets or plasticized material and fiber fractions in the mixture.
German patent publication no. DE 35 415 32 discloses an apparatus for maintaining a constant weight ratio between thermoplastics and fiber material. Rovings of fiber material are hereby arranged on a weighbridge for measuring the amount of fibers being drawn, and the determined value is then compared with a setpoint value. This comparison of setpoint value with actual value takes place in a control unit which adjusts the respective screw speed of the extruder in response to a deviation between the setpoint value and the actual value. If the actual value falls below the setpoint value, the screw speed is correspondingly increased to thereby draw in more fiber material, whereas the screw speed is correspondingly decreased, when the actual value exceeds the setpoint value. A drawback of this apparatus resides in the significant inaccuracy of the measuring process.
FIG. 2 shows a schematic illustration of the conventional apparatus according to German patent publication no. DE 35 415 32, including an extruder 1 with conventional fiber feeding device. The extruder 1 is constructed as a twin screw extruder which is operated by a motor 4. Pellets of thermoplastic material are introduced by a metering device 6 through a first inlet port 2 into the interior of the extruder 1, whereas fibers are drawn from rovings, disposed on a weighbridge 7, and introduced through a second inlet port 3 into the interior of the extruder 1. A control unit (not shown) controls the screw speed of the extruder screws in dependence on the amount of fiber drawn from the rovings.
As the fiber mass being delivered per time unit is very small in comparison to the mass of the roving(s) placed on the weighbridge, highly sensitive measurements are required. In particular, when the rovings have significant size, substantial take-off forces are necessary to draw the glass fibers. To prevent a kinking or tearing of the fibers during the delivery operation, take-off from the roving should be implemented in a substantial vertical direction. These high vertical take-off forces greatly impact the measuring results of the scale assembly. Tests have shown that the measuring value can fluctuate in a wide range so that the quality of the plastic products is directly adversely affected.
It would therefore be desirable and advantageous to provide an improved plastics processing machine to obviate prior art shortcomings and to include a scale assembly by which the amount of fiber being fed to the extruder is accurately measured to produce plastic products of high quality.